The present invention relates to an optical element and a spectroscopic device for use in an optical communication system, an optical measuring instrument, etc.
Increase in capacity of an optical fiber communication network has been intensely demanded with the rapid advance of popularization of the Internet in recent years. Development of WDM (wavelength division multiplexing) communication as means for increasing the capacity has been advanced rapidly. In WDM communication, optically functional elements such as an optical demultiplexer, a filter and an isolator excellent in wavelength selectivity are required because various kinds of information are transmitted individually by light having slightly different wavelengths. It is a matter of course that mass production, miniaturization, integration, stability, etc. are strongly required of the functional elements.
An optical demultiplexer (or a spectroscopic device) is used for demultiplexing/detecting an optical signal multiplexed with a plurality of wavelengths artificially as in wavelength division multiplexing optical communication or for spectrally analyzing target light as in spectrometry. The optical demultiplexer needs spectroscopic elements such as a prism, a wavelength filter, and a diffraction grating. Particularly, the diffraction grating is a typical spectroscopic element. For example, a silica or silicon substrate having a periodic micro prismatic structure formed in its surface is used as the diffraction grating. Diffracted light rays generated by the periodic micro prismatic structure interfere with one another, so that light at a specific wavelength exits in a specific direction. This property is used for the spectroscopic element.
A reflection diffraction grating satisfies the equation:sin θi+sin θo=mλ/din which m is the order of diffraction of the diffraction grating, d is a grating constant, λ is a wavelength used, θi is the angle between input light (an optical axis 5 of an optical fiber) and a line normal to the surface in which the diffraction grating is formed, and θo is the angle between output light and the normal line.
When the wavelength λ is changed by Δλ while θi is kept constant, the positional change Δx of each light ray which reaches an acceptance surface far by a distance L from the diffraction grating is given by the following equation.Δx=(Lm/(d·cos θo))·ΔλAccordingly, signals separated by wavelengths can be obtained if acceptance units are arranged on the acceptance surface at intervals of a positional pitch calculated in accordance with a wavelength pitch by the aforementioned equation.
An output angle from the diffraction grating, however, has little dependence on wavelength. Assume the case where light, for example, having wavelengths arranged at intervals of 0.8 nm (equivalent to a frequency pitch of 100 GHz) in a 1.55 μm-wavelength band used in optical communication needs to be demultiplexed. When the order m of diffraction is 25 in the condition that the input angle θi is 71.5° whereas the output angle θo is 38.5°, the grating constant d of the diffraction grating is 24.7 μm. The change of the output angle obtained in accordance with the wavelength pitch of 0.8 nm in this system is only about 0.06°. If the light is to be separably accepted by acceptance elements arranged at intervals of 50 μm, a distance L of 48 mm is required.
That is, generally, the positional change Δx of a light spot on the acceptance surface needs to be not smaller than the order of tens of μm because each acceptance unit has a predetermined size. Because m and d which are constants of the diffraction grating cannot be changed largely, the distance L needs to be made large in order to obtain a necessary value of Δx in accordance with a small wavelength change Δλ. Hence, there is a problem that device size cannot but become large in order to improve the performance of the optical demultiplexer using the diffraction grating.